This invention relates to new aminotriazinepolycarboxylic acids and partial esters thereof, to their preparation, and to their use as curing agents for epoxide resins.
U.S. Pat. No. 2,337,220 disclosed the reaction of mercaptans, especially aliphatic mercaptocarboxylic acids, with N-methylol or N-halomethyl derivatives of amides. Reaction between thioglycolic acid and an N-methylolamide is said to result in formation of an N-(carboxymethylthiomethyl)amide, i.e., etherification, and not esterification, occurs. The detailed disclosures are restricted to the use of N-hydroxymethylamides of aliphatic monocarboxylic acids containing at least 12 carbon atoms, but amides of many other kinds of acids and also urea, thiourea, and melamine, as well as their substitution products, are cited as possible reactants.
U.S. Pat. No. 2,763,649 described the formation of hardenable, ternary condensation products by reaction of (a) a formaldehyde condensation product of a compound of the aminotriazine or urea group or an ether thereof with an alcohol of low molecular weight, (b) an aliphatic compound which contains a carbon chain of at least 7 carbon atoms and a reactive hydrogen atom bound to a heteroatom, and (c) a hydroxy- or mercapto - carboxylic acid or a hydroxy- or mercapto-sulfonic acid. When (a) is a methylolated aminotriazine or an ether thereof, 1 mol. of (c) is employed per mol. of (a).
Neither of these references discloses the production of aminotriazinepolycarboxylic acids.
We have found that reaction of an aminotriazine containing at least two N-methylol or N-alkoxymethyl groups with at least two equivalents of a mercaptocarboxylic acid gives aminotriazinepolycarboxylic acids in which the carboxyl groups are linked to the aminotriazine ring through thioether linkages. Further, when a poly(alkoxymethylamino)triazine is used as starting material, a subsequent esterification may also occur to a minor extent, so that the product may contain not only the aminotriazinepolycarboxylic acid but also its partial esters, where the esterifying group is the same as the original etherifying group; thus, with methoxymethylaminotriazines, methyl esters of the polycarboxylic acids may be also formed. We have also found that partial esters can be obtained by replacing a proportion of the mercaptocarboxylic acid by an ester thereof, or by partial reaction of the aminotriazinepolycarboxylic acid with an alcohol.
Polycarboxylic acids are well known as curing agents for epoxide resins, i.e., substances having more than one 1,2-epoxide group per average molecule. These known acids often require a high temperature and/or extended heating in order fully to cure the resin. We have further found that the new polycarboxylic acids, and partial esters thereof which contain at least two carboxylic acid groups, cure epoxide resins on heating at moderate temperatures and for only a short period.